Handle topdown.Undefined Query result in API

If the caller GETs a completely defined virtual doc that evaluates to
undefined, the API should return an error instead of 2xx.

runtime/server.go

@@ -36,6 +36,11 @@ func (err *apiErrorV1) Bytes() []byte {
 	return nil
 }
 
+// undefinedV1 models the an undefined query result.
+type undefinedV1 struct {
+	IsUndefined bool
+}
+
 // patchV1 models a single patch operation against a document.
 type patchV1 struct {
 	Op    string      `json:"op"`
@@ -213,6 +218,11 @@ func (s *Server) v1DataGet(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if _, ok := result.(topdown.Undefined); ok {
+		handleResponseJSON(w, 404, undefinedV1{true})
+		return
+	}
+
 	handleResponseJSON(w, 200, result)
 }
 

runtime/server_test.go

@@ -52,6 +52,8 @@ func TestDataV1(t *testing.T) {
 				import req1
 				import req2 as reqx
 				g :- req1.a[0] = 1, reqx.b[i] = 1
+
+				undef :- false
 				`
 	tests := []struct {
 		note string
@@ -110,6 +112,10 @@ func TestDataV1(t *testing.T) {
 				"Message": "evaluation error (code: 1): unbound variable req2: req2.b[i]"
 			}`},
 		}},
+		{"get undefined", []tr{
+			tr{"PUT", "/policies/test", testMod, 200, ""},
+			tr{"GET", "/data/testmod/undef", "", 404, `{"IsUndefined": true}`},
+		}},
 	}
 
 	for i, tc := range tests {